Cause of communication failure in modicon quantum PLC system


Thread Starter


We are experiencing frequent communication failures for various I/O modules at our plant. The PLC system is Schneider:Modicon Quantum with remote RIO cards connected at various points in the plant.

What could be the cause for this?

Curt Wuollet

The cabling is always a good place to start with communication errors. Find out how the cabling is supposed to be done and then check how it is done.

I've found a lot of inventive and just plain wrong cabling that amazingly works most of the time. Something like Modbus will work even with one wire disconnected or grounded, just not all of the time.


I would first suspect that your RIO system cabling is the source of your communication problems. Modicon has very specific requirements for the type of cable, connectors, crimp tool used to affix the connectors, trunk/drop lengths, bend radii, etc. Too often, people take shortcuts to save money or time when building the cable system. Survey your cable system after first studying Modicon's published requirements.

If not that, perhaps a bad CRA or CRP module (I believe there was a recall on some serial numbers in years past).
I agree with the others regarding making sure you are following the recommended cabling procedures.

With that said, we had a similar problem where the communications were intermittently dropping out. We discovered that the communications cable had been routed right next to a 480VAC variable frequency drive for a vibratory feeder. Since the feeder only came on once in a while it took us some time to realize what was going on. By simply rerouting the communication cable away from the drive we were able to completely fix the issue.
Hello Mausahwrtsila
Comms errors are always difficult to fault find especially if they are sporadic. Cabling issues is a good place to start...drop lengths , cable type (usually Belding cable), screening, connection ...this is usually very specific to the supplier.
The other things are a little bit more obscure, Omron PLC if you write greedy routines this can cause all sorts of problems on the comms......
Terminating resistors ...I've had device net crashing for the sake of 1 meter difference in cable position a drop or an end cable was in question.
If you can sniff the network that is usually a good thing ...try wire shark..this has helped me in the past.
If you have a scope you then look at the signal on the cable this has also helped me in the past...if you have a fast scope ...try and see if there are reflections on the cable, noise will also stick out like a sore thumb....
A faulty device on the network can also cause problems...
If you can see a point in time when all this started ..then that might give you a clue.
I'm sure other people can add more things.....
but for sure comms issues are a nightmare to solve..